1. Field of the Invention
The present invention relates to an exposure apparatus which projects the pattern of an original onto a substrate using a projection optical system, thereby exposing the substrate.
2. Description of the Related Art
In recent years, with advances in micropatterning and an increase in the packing density of semiconductor integrated circuit devices and liquid crystal panel devices, improvements in the accuracy and function of an exposure apparatus used in lithography for manufacturing these devices are in progress. A technique of positioning an original (also called a mask or reticle) and a substrate (a wafer or glass plate) on the order of nanometers for their alignment is expected to be available.
The exposure apparatus sequentially transfers the pattern of an original onto a plurality of shot regions on a substrate while moving the substrate step by step. An exposure apparatus which performs this transfer while the original and the substrate remain still is called a stepper. An exposure apparatus which performs this transfer while scanning the original and the substrate is called a scanner or a scanning stepper.
Recently, an exposure apparatus which mounts two substrate stages has been provided in order to meet two demands, namely, improvements in overlay accuracy and throughput. Such an exposure apparatus includes an exposure station for exposing a substrate, and a measurement station for measuring a substrate. While the exposure apparatus exposes a substrate in the exposure station, it measures a substrate to be exposed next in the measurement station. This makes it possible to improve the throughput while ensuring the substrate measurement time for improving the overlay accuracy (Japanese Patent Laid-Open No. 2006-108582).
The global alignment scheme is available as a substrate positioning method. FIG. 10 is a view illustrating the arrangement of shot regions on a substrate 5. As illustrated in FIG. 10, a plurality of shot regions ST formed by preprocessing are arranged on the substrate 5. The same patterns are generally formed in all shot regions. Also, alignment marks are set in all shot regions. The substrate can be positioned by selecting shot regions (measurement shot regions) to measure the positions of the alignment marks from all these shot regions, and measuring the positions of the alignment marks in the selected shot regions.
FIG. 11 is a view illustrating measurement shot regions. For example, the positions of the alignment marks in hatched measurement shots MS are measured in FIG. 11. The arrangement information of the shot regions on the substrate can be obtained by statistically calculating the measurement value of each alignment mark. In measuring the position of the alignment mark, it is moved to the field of a detecting system and kept still in this field. This operation is performed by selecting alignment marks in the order indicated by the arrows in FIG. 11. The arrows in FIG. 11 schematically show the state in which the field of the detecting system moves relative to the substrate. The alignment marks move in directions opposite to those indicated by the arrows relative to the field of the detecting system while the field of the detecting system is fixed in position.
FIG. 12 is a flowchart illustrating the sequence of substrate positioning (alignment) measurement (alignment measurement). Step S401 is a coarse alignment process of coarsely measuring the arrangement of the shot regions. In the coarse alignment process, shot regions fewer than those used in an alignment mark sensing process and alignment mark position calculation process (to be described hereinafter) are used as the measurement targets. In the coarse alignment process, the alignment marks are sensed and their positions are measured by a detecting system having a field larger than that of a detecting system used in the alignment mark sensing process. In the coarse alignment process, the alignment marks in, two shot regions are detected.
The substrate stage is driven based on the positions of the alignment marks measured in the coarse alignment process. Step S402 is a step driving process. In this process, the substrate stage is driven so that the alignment marks fall within the field of the detecting system based on the measurement result obtained in the coarse alignment process, and are kept still in this field. Step S403 is an alignment mark sensing process. In this process, the detecting system senses alignment mark images. Step S404 is an alignment mark position calculation process. In this process, the positions of the alignment marks are precisely detected based on the sensed alignment mark images. Steps S402 to S404 are repeated until it is determined in step S405 that the alignment marks in all measurement shot regions have been measured, and the measurement process is ended.
The global alignment scheme can obtain a high throughput and high accuracy. Moreover, the global alignment scheme is convenient because it allows alignment according to the same correction scheme throughout the entire substrate region (Japanese Patent Laid-Open No. 09-218714).
As demand for alignment accuracy is becoming stricter, even error components which are conventionally too small in amount to be problematic are becoming non-negligible. Under these circumstances a proposal has been made, which improves alignment accuracy by measuring a plurality of alignment marks in a measurement shot region to calculate not only the position of the shot region but also its shape, and correcting the shape of a shot region onto which a pattern is transferred. To calculate the shape of the shot region, from the viewpoint of the accuracy to measure the alignment marks on a plurality of scribe lines set in the periphery of the shot region.
The shot magnification representing the shape of the shot region is calculated. Assuming that (two) alignment marks set on one scribe line are measured, the shot magnification can be calculated along the direction of the scribe line, but cannot be calculated in a direction perpendicular to the scribe line. This makes it necessary to use a method of estimating the shot magnification in a direction perpendicular to the scribe line from that along the scribe line. If the alignment marks on scribe lines in two orthogonal directions are measured, the shot magnifications in these two directions can be calculated.
Increasing the number of alignment marks to be measured, in turn, increases the number of driving operations of the substrate stage and the number of times it must be kept still in the field of the detecting system, which is necessary for alignment mark measurement. Therefore, the resulting measurement processing time may adversely affect the overall throughput in an exposure apparatus which mounts two substrate stages as described above.